1. Field of the Invention
This invention relates to a structure of an optical waveguide as well as an optical wave branching unit, an optical wave combining unit, an optical selector and an optical wave combining and branching unit suitable for use, for example, with a wavelength division multiplex transmission system for optical communication.
2. Description of the Related Art
In ordinary optical communication, an optical waveguide module which functions as an optical passive device which branches or combines light on an optical waveguide is used, for example, to perform bidirectional communication or single directional communication of different signals or to perform wavelength division multiplex transmission to transmit a large amount of information in communication.
FIG. 16 shows an ordinary directional coupler as an optical waveguide module described above. Referring to FIG. 16, the directional coupler shown is formed, for example, by forming a waveguide layer on a substrate using a process of production including ion etching processing, high temperature processing and so forth. The directional coupler includes a first optical waveguide 101 and a second optical waveguide 102 each formed from a core layer having a high refractive index.
The directional coupler further includes an inter-waveguide gap 103 formed from a clad layer having a low refractive index.
When light is introduced into an input end (left end in FIG. 16) of the first optical waveguide 101, the skirt of an electric field of the light extends across the inter-waveguide gap 103 to the second optical waveguide 102, as seen from electric field distributions C and D in FIG. 16. By the electric field extending to the second optical waveguide 102, an electric field is induced also in the second optical waveguide 102.
Further, as is the light propagates on the waveguide, the electric field of the light transfers such that the height of the peak of the electric field in the second optical waveguide 102 increases while the height of the peak of the electric field in the first optical waveguide 101 decreases. Finally, the full power of the electric field transfers from the first optical waveguide 101 to the second optical waveguide 102 (the length of the waveguide required for the transfer of the full power is called complete coupling length or 100% coupling length).
Since the exudation of the electric field depends upon the wavelength, the degree at which the electric field is applied depends upon the wavelength, and also the degree (coupling efficiency) at which the electric field transfers depends upon the wavelength. In particular, as the wavelength increases, the exudation of the electric field from the waveguide increases, and consequently, the coupling efficiency increases. Accordingly, as the complete coupling length decreases as the wavelength increases.
It is to be noted that, if the length of the waveguide becomes longer than the complete coupling length, the power begins to transfer to the first optical waveguide 101 on the opposite side in accordance with the length of the waveguide, and the relationship in height between the peaks of the electric fields becomes reversed. Thereafter, the foregoing is repeated.
By the way, since light propagates so as to trace the peak of the power of an electric field, if light with which the waveguide length is equal to the complete coupling length is introduced into the first optical waveguide 101, then the light is outputted from the second optical waveguide 102. Further, if the waveguide length is longer than the complete coupling length, then the light transfers back and forth between the first optical waveguide 101 and the second optical waveguide 102 in accordance with the waveguide length.
The directional coupler shown in FIG. 16 is set so that, with respect to light of a wavelength .lambda.1, the length over which the first optical waveguide 101 and the second optical waveguide 102 extend in parallel to each other is just equal to the complete coupling length. Consequently, the light of .lambda.1 is outputted by 100% from the output end of the second optical waveguide 102. On the other hand, the directional coupler is set so that, with respect to light of another wavelength .lambda.2, the length over which the first optical waveguide 101 and the second optical waveguide 102 extend in parallel to each other is equal to twice the complete coupling length. Consequently, the light of .lambda.2 is outputted from the output end of the first optical waveguide 101.
With the directional coupler having the construction described above with reference to FIG. 16, if lights .lambda.1 and .lambda.2 are simultaneously inputted to the first optical waveguide 101, then in accordance with the wavelength characteristics of them, the light .lambda.1 is outputted from the first optical waveguide 101 while the light .lambda.2 is output:ted from the second optical waveguide
It is to be noted that, by setting the directional coupler so that the light of .lambda.1 transfers back and forth between the first optical waveguide 101 and the second optical waveguide 102 by an odd number of times, the difference between the wavelengths .lambda.1 and .lambda.2 can be set to a value near to a value which is actually required.
By the way, in the directional coupler described above, the gap between the waveguides located in the proximity of each other, the width of the waveguides, the difference in refractive index between the core and the clad and so forth are factors which determine the wavelength characteristic. Above all, the gap between the waveguides is an important factor.
FIG. 17 is a sectional view taken along line A-B of FIG. 16. Since the process of production of the directional coupler includes high temperature processing, the first optical waveguide 101 and the second optical waveguide 102 of the directional coupler are inclined to be fallen in directions in which they approach each other as seen in FIG. 17. In FIG. 17, each of the first optical waveguide 101 and the second optical waveguide 102 is indicated by a broken line when it is in a normal condition, but indicated by a solid line when it is in a fallen condition. Accordingly, the directional coupler described above has a subject to be solved in that, where the first optical waveguide 101 and the second optical waveguide 102 are in such dislocated conditions as described above, the wavelength characteristic is displaced, resulting in deterioration of the light branching characteristic of the directional coupler.
In the meantime, Japanese Patent Laid-Open Application No. Heisei 6-59142 discloses another technique wherein, in order to prevent such dislocation of a waveguide as described above, a glass embankment is formed in a spaced relationship by a desired distance on the outer side of a waveguide at a coupling portion of a directional coupler or a glass connection embankment is formed in the form of a ladder on the waveguide at the coupling portion.
Also the technique disclosed in Japanese Patent Laid-Open Application No. Heisei 6-59142, however, has a subject to be solved in that light to propagate leaks a little and has some influence on the branching characteristic of light.